Various types of ventilators and respirators have heretofore been provided. Many of such ventilators and respirators have been unduly complicated and expensive, and in addition have lacked the desired versatility. For example, they have lacked means for applying a positive pressure above atmospheric against which a patient must exhale near the end of the exhalation phase and means for terminating the positive pressure against which the patient must exhale so that the patient is exposed to ambient pressure prior to initiation of the inhalation phase. They have also lacked other adjustments. In addition, it has in the past been unduly expensive to provide volume limiting capabilities which can be used in conjunction with the respirator to provide an anesthesia unit. There is therefore a need for a new and improved respirator and method which overcomes the above disadvantages.